Which of the following sentences are grammatically correct us students have a lot work to do please return book when you?

None of the sentences you provided are grammatically correct. Here's why and how to fix them:

1. "us students have a lot work to do"

* Problem: The pronoun "us" is used incorrectly. It should be "we" because it's the subject of the verb "have." Also, "work" needs to be pluralized.

* Fix: "We students have a lot of work to do."

2. "please return book when you"

* Problem: This sentence is incomplete. It needs a direct object after "return" and a time frame after "when you."

* Fix: "Please return the book when you are finished with it."

Let me know if you'd like me to explain any of these corrections in more detail!